This invention relates to an implantable device and method for long term detection and monitoring of congestive heart failure. More particularly, this invention relates to a device and method of assessing pulmonary congestive and/or systemic venous congestion.
As is known, congestive heart failure (CHF) in a patient is caused, in part, by a build-up of fluid in the lungs and body of a patient. Typically, the of build-up of fluid in the. lungs and body (i.e. edema) is a sign of failing heart circulation, for example, as described in U.S. Pat. No. 5,876,353. Accordingly, attempts have been made for detecting the occurrence of edema in the lungs, for example, using impedance monitoring. In some cases, proposals have been made to take external or internal measurements of impedance as an index of lung water content (edema). However, suitable results have not been obtained to permit long term monitoring of pulmonary and systemic venous congestion.
By way of example, U.S. Pat. Nos. 5,876,353 and 5,957,861 describe the use of an impedance monitor for discerning edema in a patient through the use of respiratory rate and direct current (DC) impedance. In particular, an impedance and respiratory monitoring circuit is added to a pacemaker system in order to obtain measurements of transthoracic impedance and respiratory rate. These measurements are taken over a long term to obtain a long term average signal so that differences can be used to algorithmically process the variance over time for use to assess the amount of tissue edema over the long term changing condition. However, such techniques fail to distinguish between left heart failure causing lung (pulmonary) edema and right heart failure causing systemic venous congestion. In clinical cases, right and left heart failure may occur in concert or independent of each other. DC impedance in particular is more reflective of systemic venous congestion and may be insensitive for detection of pulmonary edema. Furthermore, respiratory rate changes are a relatively late consequence of CHF and may not occur far enough in advance of dangerous clinical consequences to permit intervention. In addition, respiratory rate changes are not specific to CHF. They occur with exacerbations of various ling diseases such as emphysema, asthma and the like, as well as with anxiety, fever, ascent to high altitudes, and other common conditions.
U.S. Pat. No. 5,282,840 describes a multiple frequency impedance measurement system for monitoring a condition of a patient""s body tissue in order to obtain an indication of the condition of the tissue. As described, the measurement system employs a pair of electrodes which are located in contact with the tissue to be monitored. During use, electrode signals are to be applied to the two electrodes at different frequencies with the impedance between the electrodes being measured at the different frequencies. Any changes which occur in the measured impedances over time are then used to indicate changes in tissue condition, such as those induced by ischemia, drug therapies, allograft rejection, lead fractures or insulation degradation. When used with a cardiac pacemaker, the impedance values may be compared over a period of time such as hours, days, weeks or even months and may be employed to provide an increase in minimum or base pacing rate in an attempt to counteract a detected ischemia. The measurement system may also be employed to measure respiration for control of the pacing rate. In this case, one electrode is located within the right ventricle in spaced relation to a can electrode of a pacemaker with a substantial amount of lung tissue located within the sensing field of the electrodes. Changes in the impedance may then be used to calculate a respiration rate.
U.S. Pat. No. 4,676,252 describes a double indicator pulmonary edema measurement system for measuring in vivo extra vascular lung water (pulmonary edema). As described, the detection of pulmonary edema is provided by detecting the response of the pulmonary vascular network to indicator dilution of thermal and conductivity modifiers as a function of not only detected thermal and conductivity value but additional body parameters including blood characteristics and a temperature modifier of conductivity. From these, a value for extra vascular heat capacity is determined from which a quantified lung water measurement is obtained.
U.S. Pat. No. 5,003,980 describes a method and apparatus for measuring lung density by Compton Backscattering.
It is an object of the invention to provide a relatively simple technique for long term detection and monitoring of right and left sided congestive heart failure.
It is another object of the invention to assess congestive heart failure by measuring bioimpedance with an implant device.
It is another object of the invention to utilize a cardiac pacemaker, defibrillator and/or cardioverter/defibrillator with circuitry for monitoring of congestive heart failure in a patient.
Briefly, the invention provides an implantable device and method for long term monitoring of congestive heart failure. In particular, the invention provides a method of measuring bioimpedance to assess congestive heart failure and particularly lung capacitance as an index of pulmonary congestion.
The invention recognizes that the different bio-electric properties of blood and lung tissue permit separate assessments of systemic venous congestion and pulmonary congestion. That is, the lung has a high resistance to current flow as compared to venous blood with the structure of the lung being similar to a nearly dry sponge. As is known, the lung is a honeycomb of air spaces (i.e. dielectric) surrounded by blood filled capillaries and associated pulmonary arterial and venous vessels (i.e. conductors). Thus, the lung may be modeled as an array of resistors and capacitors which can be simplified to a parallel resistive-capacitive circuit. As the lung becomes congested with edema fluid, its resistance decreases and its capacitance changes as well.
Electrical impedance is a vector quantity. Vector quantities have a scalar magnitude and a direction given as an angle ("PHgr") to the horizontal. Using trigonometry, a vector can be resolved into its horizontal and vertical components. In the case of electrical impedance, the scalar magnitude of impedance (Z) is given by the Voltage across a circuit divided by the Current through the circuit. The angle ("PHgr") is the phase angle between the voltage and current. The horizontal or xe2x80x9crealxe2x80x9d component of impedance is the resistance (R) and the vertical xe2x80x9ccomplexxe2x80x9d component of impedance is the reactance (X). Reactance may be due to inductance or capacitance, but in the case of thoracic impedance, reactance is purely capacitive (Xc). Capacitive reactance is given by the formula Xc=1/(2xcfx80fC), where f is the frequency of an alternating sine wave signal, and C is the capacitance.
By measuring the capacitive component of lung impedance, an index can be obtained which is independent of the systemic venous resistane That is, there would be separate indexes of systemic venous congestion and of pulmonary congestion
Development of the implantable device is based, in part, on a realization that the blood in the right ventricle and venous system provides the lowest resistance path between an electrode in the heart and an electrode implanted in an upper chest of the patient, usually not far from the subclavian vein. The resistance of this blood path is expected to be much lower than an alternate parallel current path through aerated lung tissue. Thus, the right ventricle and systemic venous system of the upper body are likely to dominate in a resistance measurement where the low resistance of the venous system is in a parallel circuit with the high resistance lung.
With direct current, i.e. a constant unvarying signal with a constant amplitude, the capacitive reactance is infinite, so at DC and very low frequencies, almost all of the current flow is through the blood of the vascular system in general and the systemic venous system in particular. Very little current will flow through the lung because the lung has a much higher resistance than the blood in the systemic venous system. However, at higher frequencies, the capacitive reactance of the lung decreases and may be comparable in magnitude to the systemic venous resistance. A single measurement of impedance and the phase angle allows a computation in a microprocessor using conventional circuitry of the real component of impedance (resistance), which will primarily be due to current flow in the systemic venous system, and the complex component of impedance (capacitive reactance) which will reflect conditions in the lung. The lung will also have a resistive component, but it would be very high so little current will flow there and so can be considered clinically negligible.
This invention takes advantage of the fact that the physical properties of the lungs are such that they have electrical capacitance, and the physical properties of the blood are such that there is little or no capacitance at the frequencies that will be used. By modeling the systemic venous blood as purely resistive, and the lung as at least partially capacitive, one can semi-independently use resistance as an index of systemic venous congestion, and capacitance as an index of pulmonary congestion. This can be done at a single frequency by calculating the complex component (capacitive reactance) and the real component (resistance) from the total impedance and phase angle. Capacitance can be calculated from the capacitive reactance if the frequency of the signal is known.
In parallel circuits, as this model, the calculations are easier if the inverse of the quantities are used. That is to say, admittance is the inverse of impedance, conductance is the inverse of resistance and the complex (imaginary) component of admittance is the inverse of reactance.
There are several ways to measure the capacitive component but the most useful is a dual frequency measurement. In this case, the response of the circuit would be frequency dependent and would give different impedances for a high frequency sine wave versus a long DC pulse. In the event that sine waves are difficult to generate in a pacemaker device, a train of short duration pulses may be generated by the pacemaker instead. Short and long pulse widths should also yield different impedances because of their different frequency content.
For purposes of the invention, the type of high frequency signal which is used to obtain the measurements required is one which has a current whose amplitude varies with a repeating pattern at a specific frequency. This includes sine waves, square waves, truncated exponential waves which are typically the output of a pacemaker and the like.
In one embodiment, use is made of a dual frequency measurement to separate out the systemic venous congestion and the pulmonary (lung) congestion. In this respect, system venous congestion is detected using a low frequency or direct current resistance or conductance measurement of the current path through the blood of a systemic venous system. Pulmonary congestion is detected using a high frequency measurement of complex impedance, complex admittance or capacitance of the lung. As these tissues become congested, the resistance and the complex component of impedance will change and allow separate detection of systemic venous and/or pulmonary congestion.
In this embodiment, the implantable device may include a pair of electrodes which are employed in suitable locations in a patient in order to obtain the desired measurements, and, more particularly the changes in measurements over time, in the systemic venous system and in the lungs as well as a signal generator for emitting an electrical signal to the first electrode. The signal generator may be incorporated in a pacemaker which is implanted into the chest of a patient or may be employed with other implanted devices, such as defibrillators and cardioverter/defibrillators or dedicated solely to the monitoring of a patient for cardiac heart failure (CHF) as a stand-alone device.
The electrodes may be placed in the subcutaneous tissue of the chest or may be surgically implanted inside the chest as in during heart surgery or during implantation of transvenous electrodes, for example in or on the right heart, i.e. in the right ventricle or the right atrium. Also, one electrode may be implanted in the right heart while the second electrode is on the housing for the signal generator or placed elsewhere in the chest to optimize the measurements being made. In any case, a first current path between the electrodes is to be established through the blood of the systemic venous system which includes, for example, but not limited to, the right ventricle, the right atrium, the vena cava, the innominate vein and the subclavian vein. In addition, a second current path between the electrodes is to be established which passes through the lung tissue of the patient, for example from the right atrium to the signal generator or from the right ventricle to the signal generator.
Depending on the impedance offered by the body tissue, the current from the first electrode passes through the first current path extending through the blood of the systemic venous system of the patient and/or the second current path extending through the lung of the patient.
The implantable device also includes a means for measuring the resistance or the conductance to current flow in the first current path as well as for measuring at least one of complex impedance, complex admittance and capacitance in the second current path whereby changes in the measurements of the resistance or conductance are indicative of systemic venous congestion and changes in the measurements of complex impedance, complex admittance or capacitance over a period of time are indicative of pulmonary congestion. In terms of circuitry, the measuring means includes a microprocessor to receive measurements of the current, voltage and the phase angle between the current and voltage passing between the electrodes and to calculate the desired values.
The measuring means also employs means for detecting the current generated by the electrical signal and for emitting a signal indicative of the current flowing through the body of the patient to the microprocessor. An operational amplifier is also provided to amplify the signal for delivery to the microprocessor.
The measuring means also includes a second operational amplifier connected to the electrodes or to a second pair of electrodes for detecting the voltage generated by the electrical signal through the body of the patient and for emitting a corresponding amplified signal to the microprocessor.
The measuring means also includes a phase detector connected to each amplifier to measure the phase angle between the detected voltage and the indicated current.
The microprocessor is connected to the phase detector and to each amplifier to receive values corresponding to the measured phase angle, the detected voltage and the indicated current and to calculate the real and complex components of the impedance of the body tissues based upon the received values. In this respect, a calculated impedance at a direct current (DC) or at a low frequency of the electrical signal from the signal generator is indicative of systemic venous congestion while the calculated complex impedance at a higher frequency of the electrical signal from the signal generator is indicative of pulmonary congestion.
In accordance with the dual frequency measurement method of the invention, a DC signal or an electrical signal of low frequency is passed into both paths. The resistance, in this case, is dominated by the current flowing in the systemic venous system. Thus, the resistance which is measured is indicative of the systemic venous system. Should there be a decrease in measured resistance, this would be indicative of congestion of the systemic venous system.
Subsequently, an electrical signal of higher frequency is passed into the two paths and the complex impedance, complex admittance or capacitance to the current flow of the signal is measured in the second path. Should there be a change in the measured values, this would be indicative of congestion of the lung.
In another embodiment, a single frequency measurement may be used whereby the computed real component of impedance is indicative of systemic venous congestion, and the complex component of impedance, admittance or the capacitance is indicative of pulmonary congestion.
Once the microprocessor has determined that there is congestion in one or the other, or both, of the systemic venous system and the lung, signals may be emitted, for example, to a pacemaker to increase the pacing rate in order to reduce the congestion. Alternatively, the microprocessor may deliver signals to a pacemaker which may pace multiple sites in the left and right ventricle to improve the contraction pattern and function of the heart.
In other cases, the signals received from the microprocessor may also be processed for purposes of administering drugs, changing a dosage rate of drug therapy and the like. In still other cases, signals may be emitted in the form of tones or the like to alert a patient or may be emitted to a remote location to alert medical personnel.